1. Field of Application
The present invention relates to a method and apparatus for adjusting the orientation direction of an array antenna of a direction detection apparatus which transmits radar waves.
2. Description of Related Art
Types of direction detection apparatus are known in which radar waves are transmitted, to be reflected from objects located ahead of the apparatus (with such objects referred to in the following as target objects), and thereby returned to the direction detection apparatus as reflected incident waves that are received as signals by an array antenna. The array antenna is formed of a plurality of antenna elements, and the direction detection apparatus estimates the total number of these target objects, and calculates the respective directions of the target objects, based on respective received signals obtained from the antenna elements.
In addition to detecting the direction of target objects, such an apparatus can also estimate the distance to each target object, and the relative velocity of the target object (i.e., with respect to the direction detection apparatus). A specific example of such a type of apparatus is a FMCW (frequency modulation continuous wave) radar apparatus.
As shown by the full-line portions in FIGS. 8A and 8B, a FMCW radar apparatus generates a transmission signal Ss that is frequency-modulated by a triangular-waveform modulation signal, to successively linearly increase and linearly decrease in frequency within each of respective modulation periods. The part of a modulation period in which the frequency of the transmission signal Ss linearly increases is referred to in the following as the rising interval, while the part in which the frequency linearly decreases is referred to as the falling interval.
Resultant reflected radar waves are received from a target object as illustrated in FIG. 8A, with a received signal Sr being obtained as shown by the dotted-line portions in FIG. 8B. For simplicity of description, the case of a single target object as shown in FIG. 8A will be described.
As shown, the received signal Sr is delayed by the amount of time required for the radar waves to travel to and back from the target object, with that time amount indicated as Tr. In addition, the frequency of the received signal Sr is shifted by an amount fd, which is determined by the relative velocity of the target object (i.e., caused by a Doppler shift).
With a FMCW radar apparatus, the transmission signal Ss and the received signal Sr are combined in a mixer circuit to obtain a beat signal Bt (shown in FIG. 8C), whose frequency is the difference between the respective frequencies of the transmission signal Ss and the received signal Sr. The frequency obtained for the beat signal Bt during each rising interval of the transmission signal Sr will be designated as fb1, while the frequency of Bt during each falling interval will be designated as fb2.
A frequency fr that is determined by the delay time Tr can be calculated from the beat frequency values fb1 and f, using equation (1) below, while the Doppler shift frequency fd can be calculated from fb1 and fb2 by using equation (2) below.
The distance R and the relative velocity V of the target object can be calculated based on the frequency values fr and fd, by using equations (3) and (4) below. In these equations, c denotes the propagation velocity of the (electromagnetic) radar waves, fm is the modulation frequency of the transmission signal Ss, Δf is the frequency modulation depth of the transmission signal Ss, and F0 is the center frequency of the transmission signal Ss.
                    fr        =                                            fb              ⁢                                                          ⁢              1                        +                          fb              ⁢                                                          ⁢              2                                2                                    (        1        )                                fd        =                                            fb              ⁢                                                          ⁢              1                        -                          fb              ⁢                                                          ⁢              2                                2                                    (        2        )                                R        =                              c            ·            fr                                              4              ·              fm              ·              Δ                        ⁢                                                  ⁢            F                                              (        3        )                                V        =                              c            ·            fd                                              2              ·              F                        ⁢                                                  ⁢            0                                              (        4        )            
Thus with a FMCW radar apparatus, by applying Fourier transform processing to the sampled beat signal BT and performing frequency analysis of the results, the range and relative velocity of a target object can be obtained.
If the direction of incident waves (reflected from a specific target object) differs from the orientation direction (i.e. reference 0° direction) of the array antenna, then the respective received signal frequencies from the array of antenna element, and hence the corresponding beat signal frequencies, will be successively different. Hence the levels of received signal power obtained at various different frequencies, from each of the antenna elements, can be used in detecting the directions of target objects, e.g., by using the MUSIC method as described in the following. For example such a method of direction detection using an antenna formed of a plurality of antenna elements is known, whereby a correlation array is derived based on the correlation between the received signals of respective antenna elements, a direction angle spectrum is obtained from the correlation array, and the direction angle spectrum is analyzed to obtain the direction of a target object, as described in Japanese patent publication No. 2006-047282.
The MUSIC method is widely known as a method of detecting the direction of a target object, and its principles (as applied to an embodiment of the present invention) can be summarized as follows. It will be assumed that as shown in FIG. 2, an array antenna is utilized having M antenna elements that are disposed at equidistant intervals along a straight line. For each of the received signals (CH_1˜CH_M) from the antenna elements, the values of signal power and frequency that are obtained (from beat signal BT samples) during a modulation period are operated on by applying the Fourier transform to obtain a spectrum that relates signal power to frequency. FIG. 9A shows an example of the respective spectrums thereby obtained for signals CH_1˜CH_M, assuming a case in which incident reflected waves are received from a single direction. As shown, this results in a common peak in the spectrums (i.e., with respectively different values of center frequency of the common peak being obtained by the signal channels CH_1˜CH_M). In general, there may be a plurality of such common peaks, corresponding to respectively different incident wave directions, i.e., corresponding to respective target objects.
With the MUSIC method, for each such common peak, the Fourier transform is obtained of the values (center frequency, signal power) of the common peak respectively obtained from the various channel signals CH_1˜CH_M. An array of M Fourier transform values is thereby obtained, which can be expressed as an M-dimensional received signal vector X, as in equation (5) below. By using this received signal vector X, a correlation array Rxx having M rows and M columns can be obtained, expressed by equation (6) below.X=(x1,x2, . . . , xM)T  (5)Rxx=XXH  (6)
The term Xm (where m=1, . . . , M) of the received signal vector X is the Fourier transform value of the signal obtained for the m-th signal channel (i.e., from the m-th antenna element) corresponding to a common peak, and is a complex value. In equations (5), (6), T denotes the vector transpose, and H denotes the complex conjugate transpose.
After deriving the correlation array Rxx, the eigenvalues λ1˜λm of the correlation array Rxx are derived (where λ1≧λ2≧ . . . λm), and an incident wave number L (i.e., estimated number of different directions from which reflected waves are incident on the array antenna) is deduced from the number of the eigenvalues that are greater than thermal noise power level. In addition, the eigenvectors e1˜em corresponding to the eigenvalues λ1˜λm are calculated.
In general, the correlation array Rxx will be compensated, based on time-domain averaging or spatial-domain averaging of that correlation array, to obtain a compensated correlation array Rxx′. The eigenvalues λ1˜λm of the correlation array Rxx′ are then derived, and the incident wave number L is deduced from the number of these eigenvalues that exceed thermal noise power level, with the eigenvectors e1˜em corresponding to these eigenvalues λ1˜λm then being calculated.
After calculating the eigenvectors e1˜em, a noise eigenvector EN is defined, using equation (7) below. The noise eigenvector EN corresponds to the set of (M-L) eigenvalues that do not exceed thermal noise power level. An evaluation function Pmu(Θ) is then obtained, from equation (8) below, in which a (Θ) denotes the complex response of the array antenna with respect to the direction Θ, where Θ is defined as an amount of angular displacement from the orientation direction of the array antenna.
                              E          N                =                  (                                    e                              L                ⁢                                                                  +                1                                      ,                          e                              L                +                2                                      ,            …            ⁢                                                  ,                          e              M                                )                                    (        7        )                                          P          MU                =                                                            a                H                            ⁡                              (                θ                )                                      ⁢                          a              ⁡                              (                θ                )                                                                                        a                H                            ⁡                              (                θ                )                                      ⁢                          E              N                        ⁢                          E              N              H                        ⁢                          a              ⁡                              (                θ                )                                                                        (        8        )            
As shown in FIG. 9B, a direction angle spectrum (MUSIC spectrum) is obtained from the evaluation function Pmu(Θ). Such a direction angle spectrum has a single sharp peak, which coincides with a specific direction theta of incident radar waves, i.e., the direction of the incident waves that resulted in the common peak shown in FIG. 9A.
When a plurality of common peaks (each as illustrated in FIG. 9) are concurrently detected, then each of the peaks is selected in turn, to derive a corresponding direction angle spectrum of the form shown in FIG. 9B.
Hence a number of different directions from which incident radar waves are arriving can be derives as respective sharp peaks in a MUSIC spectrum. Thus (on the assumption that the estimated wave number value L actually corresponds to the number of target objects), the respective directions of the target objects in relation to the orientation direction of the antenna can be calculated.
When such a type of radar apparatus is mounted on a vehicle, it is necessary to attach the radar apparatus to the vehicle such that the orientation direction (as defined above) of the antenna is accurately aligned with the direction of motion of the vehicle, i.e., coinciding with the longitudinal central axis of the vehicle. This is due to the fact that the radar apparatus derives the direction of a target object by using coordinates which have the orientation direction of the antenna as a reference. If the relationship between this coordinate system and the vehicle position coordinates is not fixedly predetermined, then it is not possible to use the vehicle as a reference for evaluating the detected position and direction, etc., of a target object, in order to control the vehicle in accordance with such detection information.
Hence it has been usual practice, when installing a radar apparatus on a vehicle, to position a target object consisting of a radar reflector ahead of the vehicle, aligned with the central longitudinal axis of the vehicle. The orientation of the array antenna of the radar apparatus is then adjusted until the detected direction that is obtained for the radar reflector by the radar apparatus coincides with the actual known direction, i.e., so that the detected direction of the reflector becomes 0°.
However in practice, the direction detection results that are obtained from such a type of apparatus are affected by factors such as electrical noise, etc., causing the detected direction obtained for a stationary target object to vary substantially with time. Hence the problem arises with such an adjustment method that, if the antenna orientation is adjusted while observing the direction detection results (i.e., to attempt to adjust the detected direction to coincide with the actual known direction of a target object such as a radar reflector), it is difficult or impossible to perform the adjustment to a sufficiently high degree of accuracy.